Computations and circle diagrams:Three-phase A.C. Commutator Motors

Three-phase A.C. Commutator Motors

Such motors have shunt speed characteristics i.e. change in their speed is only moderate, as compared to the change in the load. They are ideally suited for drives, requiring a uniform accelerating torque and continuously variable speed characteristics over a wide range. Hence, they find wide use in high-speed lifts, fans and pumps and in the drives for cement kilns, printing presses, pulverised fuel plants, stokers and many textile machines. Being more complicated, they are also more expensive than single-speed motors. Their efficiency is high over the whole speed range and their power factor varies from low value at synchronous speed to unity at maximum (supersynchronous) speed.

The speed control is obtained by injecting a variable voltage at correct frequency into the secondary winding of the motor via its commutator. If injected voltage assists the voltage induced in the secondary winding, the speed is increased but if it is in the opposing direction, then motor speed is reduced. The commutator acts as a frequency changer because it converts the supply frequency of the regulating voltage to the slip frequency corresponding to the speed required.

Following are the two principal types of such motors :

(i) Schrage or rotor-fed or brush shift motor and (ii) stator-fed or induction-regulator type motor.

Schrage Motor*

It is a rotor-fed, shunt-type, brush-shifting, 3-phase commutator induction motor which has built- in arrangement both for speed control and power factor improvement. In fact, it is an induction motor with a built-in slip-regulator. It has three windings:two in rotor and one in stator as shown in Fig. 35.41 and 35.42 (a) The three windings are as under:

(i) Primary winding. It is housed in the lower part of the rotor slots (not stator) and is supplied through slip-rings and brushes at line frequency. It generates the working flux in the machine.

(ii) Regulating winding. It is variously known as compensating winding or tertiary winding. It is also housed in rotor slots (in the upper part) and is connected to the commutator in a manner similar to the armature of a d.c. motor.

(iii) Secondary winding. It is contained in the stator slots, but end of each phase winding is connected to one of the pair of brushes arranged on the commutator. These brushes are mounted on two separate brush rockers, which are designed to move in opposite directions relative to the centre line of the corresponding stator phase (usually by a rack and pinion mechanism). Brushes A 1, B1, and C1 move together and are 120 electrical degrees apart. Similarly, brushes A 2, B2 and C2 move together and are also 120 electrical degrees apart. A sectional drawing of the motor is shown in Fig. 35.44.

(a) Working

When primary is supplied at line frequency, there is transformer action between primary and regulating winding and normal induction motor action between primary and secondary winding. Hence, voltage at line frequency is induced in the regulating winding by transformer action. The commutator, acting as a frequency changer, converts this line-frequency voltage of the regulating winding to the slip frequency for feeding it into the secondary winding on the stator. The voltage across brush pairs A 1A 2, B1B2 and C1C2 increases as brushes are separated. In fact, magnitude of the voltage injected into the secondary winding depends on the angle of separation of the brushes A 1 and A 2, B1 and B2 and C1 and C2. How slip-frequency e.m.f. is induced in secondary winding is detailed below:

When 3-f power is connected to slip-rings, synchronously rotating field is set up in the rotor core. Let us suppose that this field revolves in the clockwise direction. Let us further suppose that brush pairs are on one commutator segment, which means that secondary is short-circuited. With rotor still at rest, this field cuts the secondary winding, thereby inducing voltage and so producing currents in it which react with the field to produce clockwise (CW) torque in the stator. Since stator cannot rotate, as a reaction, it makes the rotor rotate in the counterclockwise (CCW) direction.

Suppose that the rotor speed is N rpm. Then

1. rotor flux is still revolving with synchronous speed relative to the primary and regulating winding.

2. however, this rotor flux will rotate at slip speed (Ns - N) relative to the stator. It means that the revolving rotor flux will rotate at slip speed in space.

3. if rotor could rotate at synchronous speed i.e. if N = Ns, then flux would be stationary in space (i.e. relative to stator) so that there would be no cutting of the secondary winding by the flux and, consequently, no torque would be developed in it.

As seen from above, in a Schrage motor, the flux rotates at synchronous speed, relative to rotor but with slip speed relative to space (i.e. stator), whereas in a normal induction motor, flux rotates synchronously relative to stator (i.e. space) but with slip speed relative to the rotor. (Art. 34.11).

Another point worth noting is that since at synchronous speed, magnetic field is stationary in space, the regulating winding acts as a d.c. armature and the direct current taken from the commutator flows in the secondary winding. Hence, Schrage motor then operates like a synchronous motor.

(b) Speed Control

It is quite easy to obtain speeds above as well as below synchronism in a Schrage motor. As shown in Fig. 35.42 (b) (i) when brush pairs are together on the same commutator segment (i.e. are electrically connected via commutator), the secondary winding is short-circuited and the machine Fig. 35.42

operates as an inverted* plain squirrel-cage induction motor, running with a small positive slip. Parting the brushes in one direction, as shown in Fig. 35.42 (b) (ii) produces subsynchronous speeds, because in this case, regulating voltage injected into the secondary winding opposes the voltage induced in it from primary winding. However, when movement of brushes is reversed and they are parted in opposite directions, the direction of the regulating voltage is reversed and so motor speed increases to super synchronous (maximum) value, as shown in Fig. 35.41 (b) (iii) The commutator provides maximum voltage when brushes are separated by one pole pitch.

No-load motor speed is given by N @ Ns (1 - K sin 0.5b) where b is brush separation in electrical degrees and K is a constant whose value depends on turn ratio of the secondary and regulating windings.

Maximum and minimum speeds are obtained by changing the magnitude of the regulating voltage. Schrage motors are capable of speed variations from zero to nearly twice the synchronous speed, though a speed range of 3:1 is sufficient for most applications. It is worth noting that Schrage motor is essentially a shunt machine, because for a particular brush separation, speed remains approximately constant as the load torque is increased as happens with dc shunt motors (Art 29.14).

(c) Power Factor Improvement

Power factor improvement can be brought about by changing the phase angle of the voltage injected into the secondary winding. As shown in Fig. 35.43, when one set of brushes is advanced more rapidly than the other is retarted, then injected voltage has a quadrature component which leads the rotor induced voltage. Hence, it results in the improvement of motor power factor. This differential movement of brush sets is obtained by coupling the racks driving the brush rockers to the hand wheel with gears having differing ratios. In Schrage motor, speed depends on angular distance between the individual brush sets (A 1 and A 2 in Fig. 35.41) but p.f. depends on the

angular positions of the brushes as a whole.

(d) Starting

Schrage motors are usually started with brushes in the lowest speed position by direct-on contactor starters. Usually, interlocks are provided to prevent the contactor getting closed on the line when brushes are in any other position. One major disadvantage of this motor is that its operating voltage is limited to about 700 V because a.c. power has to be fed through slip-rings. It is available in sizes upto 40 kW and is designed to operate on 220, 440 and 550 V. It is ordinarily wound for four or six poles.

Fig. 35.44 shows a sectional drawing of a Schrage motor. The details of different parts labelled in the diagram are as under:

1. rotor laminations 2. stator laminations 3. primary winding

4. secondary winding 5. regulating winding 6. slip-ring unit

7. commutator 8. cable feed for outer brush yoke 9. cable feed for inner brush yoke 10. hand wheel.

Motor Enclosures

Enclosed and semi-enclosed motors are practically identical with open motors in mechanical construction and in their operating characteristics. Many different types of frames or enclosures are available to suit particular requirements. Some of the common type enclosures are described below:

(i) Totally-enclosed, Non-ventilated Type

Such motors have solid frames and end- shields, but no openings for ventilation. They get cooled by surface radiation only (Fig. 35.45). Such surface-cooled motors are seldom furnished in sizes above two or three kW, because higher ratings require frames of much larger sizes than fan-cooled motors of corresponding rating.

(ii) Splash-proof Type

In the frames of such motors, the ventilating openings are so constructed that the liquid drops or dust particles falling on the motor are coming towards it in a straight line at any angle not greater than 100º from the vertical are not able to enter the motor either directly or by striking and running along the surface.

(iii) Totally-enclosed, Fan-cooled Type

In such motors (Fig. 35.46), cooling air is drawn into the motor by a fan mounted on the shaft. This air is forced through the motor between the inner fully-enclosed frame and an outer shell, over the end balls and the stator laminations and is then discharged through openings in the opposite side. An internal fan carries the generated heat to the totally enclosing frame, from where it is conducted to he outside. Because of totally enclosing frame, all working parts are protected against corrosive or abrasive effects of fumes, dust, and moisture.

(iv) Cowl-covered Motor

These motors are simplified form of fan-cooled motors (Fig. 35.47). These consist of totally-enclosed frame with a fan and cowl mounted at the end opposite to the driving end. The air is drawn into the cowl with the help of fan and is then forced over the frame. The contours of the cowl guide the cooling air in proper directions. These motors are superior to the usual fan-cooled motors for operation in extremely dusty atmosphere i.e. gas works, chemical works, collieries and quarries etc. because there are no air passages which will become clogged with dust.

(v) Protected Type

This construction consists of perforated covers for the openings in both end shields

(vi) Drip-proof Motors

The frames of such motors are so constructed that liquid drops or dust particles, falling on the machine at any angle greater than 15º from the vertical, cannot enter the motor, either directly or by striking and running along a horizontal or inwardly inclined smooth surface (Fig. 35.51).

(vii) Self (Pipe) Ventilated Type

The construction of such motors consists of enclosed shields with provision for pipe connection on both the shields. The motor fan circulates sufficient air through

(viii) Separately (Forced) Ventilated Type hese motors are similar to the self-ventilated type except that ventilation is provided by a separated blower.